As such a drill bit insert, a drill bit insert having: an insert body made of a cemented carbide; and a hard layer made of a sintered compact of polycrystalline diamond harder than the insert body and coated on a tip portion of the insert body in order to increase the tool life of a bit for percussion drilling, is known. For example, U.S. Pat. No. 4,694,918 proposes a drill bit insert having: an insert body including a cylindrical rear end portion and a hemispherical tip portion with the outer diameter reduced toward a tip side; and many layers of hard layer of the polycrystalline diamond compact coated on the tip portion of the insert body. U.S. Pat. No. 6,651,757 proposes a polycrystalline diamond compact to which a carbide such as WC is added so as to adjust a hardness thereof.